As a solid-state imaging unit, a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor that reads signal charge accumulated in a photodiode serving as a photoelectric conversion device via a MOS (Metal-Oxide-Semiconductor) transistor has been used for various applications.
In such a solid-state imaging unit, when a saturated charge amount of the photodiode is increased, the saturated charge amount may be larger than a maximum charge amount that is allowed to be accumulated in a floating diffusion section that converts the charge amount into a voltage signal in some cases. In this case, all of the charge amount accumulated in the photodiode is not allowed to be transferred to the floating diffusion section in one reading operation. Therefore, the reading operation is performed for a plurality of times (for example, see Patent Literature 1 and 2).
For example, in Patent Literature 1, first, the signal charge is divided to be transferred in a plurality of times of transfer operations by a transfer transistor from the photodiode to the floating diffusion section. It is to be noted that, at this time, an intermediate voltage is applied to a gate of the transfer transistor in the mid-course transfer operations (intermediate transfer operations). Further, in Patent Literature 1, a plurality of signals divided to be read in the plurality of times of operations are synthesized, and thereby, a voltage signal corresponding to the signal charge accumulated in the photodiode is generated.
Moreover, Patent Literature 2 discloses a method of controlling, by feedback control, the intermediate voltage at the time of dividing and reading, in the plurality of times of operations, the signal charge accumulated in the photodiode to have an optimum value. In the method disclosed in Patent Literature 2, part of pixels in a pixel section are forcibly saturated, and then, the intermediate voltage is controlled to have the optimum value based on a saturated charge amount of the forcibly-saturated pixels and charge amount remained in the photodiode after the intermediate transfer operation (mid-course reading operation).